I've read that it's possible, but it's a case by case scenario dependant on the personality of the individual fish.

You can try it, but monitor them, and have a place to separate them to, if it doesn't 'work'.

I've actually been considering this route - I have a 2 juvenile girl gourami/rainbows tank - that I'm intending to attempt to add a king betta to. (but not for another week or so) - the betta is still in quarentine - and, I'm still working on (planting) the tank to make it a bit more 'private' for the fish who require solitary time.

Alright....of course I went with what others said at petsmart (not bashing) but they kept telling me that I needed fish that basically had the same body shape as the female bettas. Whether or not that is true, I dont really know. But my females seem to be doing fine with the other fish, and as of now my gourami is NOT the community tank because I was afraid he would get killed. So I suppose I will try it today or tomorrow and see how it goes. My females are pretty laid back, especially my small white female...however I do have one female that is more protective of herself but she is in her own tank as of now due to the females fighting over their picking order and shes damaged her fins.
If anyone has any more details or info about my question I'd love to find out ASAP pleassseee.....